1. Field of Invention
The present invention relates to nonrecursive digital filters using shift registers, such as matched filters, used for cell-synchronization capturing and demodulation-synchronization capturing in spread spectrum communication systems and CDMA communication systems, and radio receiving units using the nonrecursive digital filters.
2. Description of Related Art
Japanese Unexamined Patent Application Publication No. Hei-10-178386 discloses a matched filter serving as a nonrecursive digital filter in a conventional CDMA system.
In this conventional system, packets to which spectrum spreading modulation has been applied by the use of spreading codes are transmitted, they are received by an antenna and demodulated by a receiving demodulation section. The received signal is spectrum-spreading-modulated by the use of the spreading codes, and the received signal is sent to a matched filter. This matched filter is formed of a shift register for receiving and shifting the received signal, having, for example, 64 bits; a register for setting a spreading-code sequence having the same number of bits as the shift register; a multiplier for multiplying the outputs of the shift register and the register bit by bit; and an adder for adding the output signals of the multiplier. The received signal is formed of packets and includes a preamble section and a data section. The received signal is sampled, for example, at the chip period of the spreading codes, and is converted to digital values falling between −1.0 and +1.0 by A/D conversion. The shift register shifts the received signal at the chip period according to the sampling period of the received signal. In general, to increase the precision of a correlated value, a structure is used in which the received signal is over-sampled, that is, sampled at a period shorter than the chip period of the spreading codes, and is A/D converted, and the shift register shifts the received signal according to the sampling period.